1. Field of the Invention
This invention relates generally to modulator drivers and, more particularly, to bias control of an array of low power modulator drivers provided on a single substrate.
2. Description of the Related Art
Modulator drive circuits have become commonplace in telecommunication systems as the need for increased network carrying capacity continues to grow. Generally, such a modulator drive circuit accepts a data signal as an input, e.g. a data signal to be transmitted over a network infrastructure, and generates an output modulator drive signal to be provided to a modulator, e.g. a Mach Zehnder modulator (MZM) or a semiconductor electro-absorption modulator (EAM). In response to the output modulator drive signal the modulator then modulates an optical carrier to facilitate optical transmission of the data signal across the network infrastructure. The modulator drive circuit output signal provided to the modulator generally comprises two signal components, a first signal component which represents the data signal and a second signal component which is used to bias the modulator to ensure that the modulator is operating to efficiently modulate the optical carrier. As such the first signal component is typically an alternating voltage signal or AC component of the output modulator drive signal, and the second signal is typically a steady-state voltage signal or DC component of the output modulator drive signal.
With reference to FIG. 1, a first known modulator drive circuit 100, which utilizes a bias circuit in the signal path, is shown. More specifically, drive circuit 100 comprises a modulator driver 104, and a bias circuit 106 located external to the modulator driver 104 and in the signal path of a modulator driver output signal 107. The modulator driver 104 is configured to accept an input data signal 102 labeled “Vin” in FIG. 1, typically a differential signal as depicted, at an input of the modulator driver 104 and generate the output signal 107 in response to the received input data signal 102, the output signal 107 provided to the bias circuit 106. As should be apparent to one of ordinary skill in the art, the primary purpose of the modulator driver 104 of FIG. 1 is to transform the differential input signal 102 into a single-ended signal having a common voltage reference with respect to a modulator 110. The modulator driver 104 is powered via a fixed voltage differential power supply which provides an upper fixed voltage of V+ and a lower fixed voltage of V− to modulator driver 104, as shown. The fixed voltage supply provides a suitable voltage range, V+-V−, to accommodate the amplitude of the differential input data signal 102 such that the amplitude remains within the voltage range without clipping.
The bias circuit 106, often referred to as a bias tee circuit, comprises a capacitor 106C, an inductor 106L, and an adjustable DC bias voltage input, labeled DC BIAS in FIG. 1. The output signal 107 of the modulator driver 104 is AC coupled through the capacitor 106C such that the AC component of the modulator driver output signal 107 is on to the modulator 110 as the AC component of an input signal 108. In addition to passing the AC component of the output signal 107, capacitor 106C correspondingly prevents any DC voltage which may exist as part of output signal 107. The DC BIAS voltage is coupled through the inductor 106L and passed on to the modulator 110, the capacitor 106C also preventing the DC bias voltage from entering the modulator driver 104. The DC bias voltage is provided as the DC component of the input signal 108, as is known in the art.
Such a circuit 100, however, has several drawbacks. First, the component count of modulator drive circuit 100 is high, especially when a plurality of such circuits 100 are formed as part of an integrated circuit on a single substrate. The component count can further increase since the inductor 106L, as part of the bias circuit 106 formed on a single substrate, is typically fabricated from a plurality of inductors arranged in series or parallel to achieve a desired broadband response, for example a first inductor having a small inductance and a high frequency response arranged in series with a second inductor having a large inductance and a small frequency response. Second, the size of the modulator drive circuit 100 is physically large when compared to other solutions which do not incorporate bias circuits, such as the bias tee circuit 106 provided in the signal path of the modulator driver 104 output signal 107. The components of the bias circuit 106, capacitor 106C and inductor 106L, occupy a large physical size on a semiconductor chip. Additionally, this problem is exacerbated by the fact that the modulator drive circuit 100 may be one of a plurality of modulator drive circuits, where it is desired to provide the plurality of modulator drive circuits on a single substrate as part of a semiconductor integrated circuit chip. Moreover, it is desirable to reduce the distance of the signal path between the modulator driver 104 and the modulator 110 to correspondingly reduce, or otherwise minimize, the associated transmission line effects.
With reference to FIG. 2, another alternative known general approach to providing the DC BIAS voltage signal component, or DC component, of a modulator driver output signal to a modulator is depicted. Modulator drive circuit 200, while it does not include an external bias tee in the signal path of the modulator driver output signal, utilizes a large modulator driver power supply to generate the DC BIAS voltage. Similar to the modulator circuit 100 of FIG. 1, modulator circuit 200 comprises a modulator driver 204 which accepts a data signal 202 at an input to the modulator driver 204. Unlike modulator driver circuit 104, however, modulator driver circuit 204 accepts a DC bias voltage control signal, labeled as “DC BIAS Control” in FIG. 2. The DC bias voltage is derived by circuitry, as part of the modulator driver 204, from the DC bias voltage control signal. Therefore, rather than the DC bias voltage signal provided in the signal path of the modulator signal output, the DC BIAS voltage is derived from a DC bias voltage control signal provided to the driver circuit 204. Based upon the derived DC BIAS voltage, the driver circuit 204 then provides the DC component of a modulator driver output signal 208, provided at an output of modulator driver 204. Typically, the DC bias voltage is added to the AC component of the modulator driver output 208 derived from the input data signal 202 through the use of summing circuits, comprising operational amplifiers for example, as is known in the art.
The modulator driver 204 is powered via a fixed voltage differential power supply which provides an upper fixed voltage of V+ and a lower fixed voltage of V− to modulator driver 204, as shown. Therefore, in addition to the DC bias voltage input provided to the modulator driver 204, the fixed voltage supply is provided to power the circuitry of the modulator driver 204 itself. While the approach of modulator drive circuit 200 allows for positioning the modulator driver 204 closer to a modulator 210, the modulator driver circuit 200 requires the fixed voltage power supply to supply a high voltage to the modulator driver 204. More specifically, the modulator driver 204 fixed voltage power supply must be able to provide a sufficient power and voltage range to accommodate the peak-to-peak voltage range of the AC component of the modulator driver 204 output signal 208, in addition to the voltage range of the DC component of the modulator driver 204 output signal 208. This leads to high power dissipation in the modulator driver 204, which can lead to thermal related problems especially where the modulator driver circuit is positioned in close proximity to modulated optical sources.
Furthermore, where it is desirable to provide for control electronics on the same substrate as the modulator drive circuit 200, e.g. on the same semiconductor chip, the modulator drive circuit 200 configuration restricts the integrated chip technologies which can be used for fabrication. For example, silicon-based integrated circuit technologies have low breakdown voltage requirements and, therefore, cannot easily accommodate the large modulator driver supply voltage.
What is needed is a high-speed, low power modulator drive circuit which generates a proper modulator drive signal, having both AC and DC components, through the use of an output bias voltage control without the need for a bias tee formed in the signal path of the modulator drive output signal. Further, what is needed is a high-speed, low power modulator drive circuit which does not require application of a DC bias voltage as a separate input directly to the modulator driver circuit. Additionally, what is needed is a controller for controlling the power applied to the modulator driver, as part of the modulator drive circuit, to provide control over the DC component of the modulator drive signal, as well as other operating characteristics of the modulator drive circuit. Last, what is needed is the ability to provide an array of such modulator driver circuits, along with the associated control circuitry, on the same substrate.